1. Field of the Invention
The present invention relates to improvement in the cetane rating of diesel fuel by the addition of a small but effective amount of the nitric acid ester of 1-phenyl ethanol herein called methyl benzyl alcohol nitrate (MBAN).
2. Description of the Prior Art
Diesel engines operate by compression ignition. They have compression ratios in the range of 14:1 to 17:1 or higher and for this reason obtain more useful work from a given amount of fuel compared to an Otto cycle engine. Historically, diesel engines have been operated on a petroleum-derived liquid hydrocarbon fuel boiling in the range of about 300.degree.-750.degree. F.
One major factor in diesel fuel quality is cetane number. Cetane number is related to ignition delay after the fuel is injected into the combustion chamber. If ignition delays too long, the amount of fuel in the chamber increases and upon ignition results in a rough running engine and increased smoke. A short ignition delay results in smooth engine operation and decreases smoke. Commercial petroleum diesel fuels generally have a cetane number of about 40-55.
Through the years, many types of additives have been used to raise the cetane number of diesel fuel. These include peroxides, nitrites, nitrates, nitrosocarbonates and the like. Alkyl nitrates such as amyl nitrate, hexyl nitrate and mixed octyl nitrates have been used commercially with good results.
Two main factors determine the commercial potential of a compound for use as a cetane improvement additive. These are the ease of handling the material and the cost/performance ratio relative to other suitable materials. Many nitrate esters have been described that are more effective cetane improvement additives than both the nitrate ester described in this invention and the commonly used commercial material, 2-ethylhexyl nitrate. However, most of these materials are too unstable to be of use in typical commercial applications. U.S. Pat. Nos. 4,473,378 and 4,536,190 teach that 2-methyl-2-nitropropanol nitrate requires stabilization do to its shock-sensitive nature. The shock sensitivity of this compound is typical of dinitrates from glycols. This shock sensitivity makes this compound unsuitable for commercial use. U.S. Pat. No. 4,705,534 teaches that the dinitrates prepared from a series of ethyl glycols are also unstable and require stabilization. While various stabilization techniques can be used to stabilize these nitrates, they are still considered too unstable for commercial use. Mononitrates are typically more stable than the polynitrates, therefore are more suitable for commercial applications.
Numerous mononitrates have been reported that produce a larger increase in cetane number than the nitrate ester described in this invention. However, these compounds are prepared from alcohols more costly than a-phenylethyl alcohol, which is used to prepare the nitrate of this invention. Examples of such compounds are 4-morpholine ethanol nitrate (U.S. Pat. No. 4,421,552), tetrahydro-2H-pyran-3-ol nitrate (U.S. Pat. No. 4,405,333), 1-methyl-3-piperidinol nitrate (U.S. Pat. No. 4,405,334), 1,3-dioxolane-4-methanol nitrate (U.S. Pat. No. 4,457,763) and tetrahydrofuranol nitrate (U.S. Pat. No. 4,406,665). Due to its lower cost, the nitrate ester described in this invention has a better performance to cost ratio than the nitrate esters described here.
The performance of mononitrate esters prepared from simple alkanols is similar to the performance of the nitrate ester described in this invention. The increase in cetane number obtained by adding 1.5% of some of these nitrate esters to a base diesel fuel are listed below:
______________________________________ Butyl Nitrate 16.8 Isopropyl Nitrate 17.9 Primary Amyl Nitrate 13.0 Secondary Hexyl Nitrate 17.6 n-Heptyl Nitrate 14.8 n-Nonyl Nitrate 13.3 ______________________________________
U.S. Pat. No. 4,723,963 describes improving the cetane rating of diesel fuels by adding to the fuels alkyl aromatics having an oxygenate functionality in the benzylic position. We are not aware of prior art showing the use of nitrate esters of such materials in diesel fuel.
The preparation of methyl benzyl alcohol nitrate is known. One such preparation involves the reaction of the methyl benzyl halide with silver nitrate. See Olah, G. A.; Malhotra, R; Narang, S. C.; "Nitration, Methods and Mechanisms", VCH Publishers, New York, p. 270 (1989), Heggs, B; J. Chem. Soc., 1955, 616, and Komblum, N.; Hardies, D. E.; J. Amer. Chem. Soc., 88, 1707 (1966).
The nitric acid ester can also be produced from the corresponding alcohol, 1-phenyl ethanol (methyl benzyl alcohol) using either N-nitropyridinium tetrafluoroborate or N-nitrocollidium tetrafluoroborate. See Olah, G. A.; Malhotra, R.; Narang, S. C.; "Nitration, Methods and Mechanisms", VCH Publishers, New York, p. 271 (1989), and Olah, G.; Narang, S. C.; Pearson, R. L.; Cupas, C. A.; Syn, 1978, 452.
Commercial processes for the production of nitric acid esters from corresponding alcohols typically use a mixture of nitric acid and sulfuric acid.